Breadcrumbs

Mean sea level

The long term changes in "mean" sea level provide basic information on the climatic role of oceans and general ocean circulation. The international scientific community is conducting studies to detect long-term trends and link them to climate change. But the global network of tide stations has significant shortcomings: the stations are not evenly distributed (concentrated in the northern hemisphere) and they are ill-suited to monitoring the general dynamics of the oceans. To achieve this goal, specific tide gauges must be installed and used in conjunction with highly precise satellite positioning systems (GPS, DORIS), which will monitor the geodesic stability of the base that the gauges are based on.

It is generally accepted that the global sea level is rising by about 1 to 2 mm per year, but estimates of the rate of increase differ significantly according to the authors. This is because the exact causes of the rise are not known, and in particular, what portion can be attributed to the greenhouse effect, due to the increase of industrial carbon dioxide in the atmosphere. The Topex Poseidon and Jason satellites measured a mean rise of around 2 mm per year between 1993 and 2000, but this increase is not uniform throughout the world's oceans: there are regional differences of ± 20 mm / year.

To address this issues, tides gauges must be positioned in a unique reference system, the reference ellipsoid for example. This is why tide stations are increasingly equipped with geodetic positioning systems using satellite technology.

Observations

Tidal observations

For the purposes of climate study, long-term tidal observations are valuable, especially those of Brest, which began in 1807. With nearly 200 years of measurements, the Brest tide station has the long-term data required to determine long-term variations in sea level.

This example shows that local fluctuations of annual mean sea level are relatively significant, reaching or exceeding ± 5 cm from one year to another. This is why accurate trend assessments require observation periods on the order of 100 years.

In Brest, the mean sea level has risen some 20 centimetres since 1846. But this example is not representative of all long-term observations available across the globe. In most cases, these observations show an average increase of about 1 to 2 mm / year, with a spread around these values of the same order as that of Brest. However, there are sites, especially in Scandinavia, where the trend is reversed.

The available tidal observations are not a good indicator of the overall trend due to the uneven distribution of tide stations, located mostly in the temperate regions of the northern hemisphere. The variability of the trend from one site to another is mainly due to vertical movements of the earth's crust, which the gauges obviously cannot detect (measurement of the relative level). These tectonic movements can be measured using satellite technology that can locate various levels (marine or terrestrial) compared to an absolute reference.

Altimetry observations

The data acquired by numerous satellites equipped with radar altimeters providing near-centimetre accuracy (Topex-Poseidon since October 1992, and Jason since 2003 and Jason 2 since 2008), indicate a rise in sea levels of the same order of magnitude as those mentioned above. However, because of interannual fluctuations, many years of altimetric data across the globe are needed to reduce the uncertainty of this estimate. The available satellite data show a high spatial variability of the trends over periods on the order of a decade. However, the reliability of the radar altimeters on board satellites has been demonstrated and the quality of the data is improving. Moreover, space geodesy technology can position specific points on the surface of the Earth with centimetre accuracy in the geocentric reference system International Terrestrial Reference System (IRTS), which has been adopted by the International Union for Geodesy and Geophysics (IUGG). Operational systems such as GPS and Doris can reference the tide gauge datums to the ITRS.

This makes it possible to monitor changes in sea level against an absolute reference. International and national programs to improve the accuracy of reference systems and geoid models from these satellite technologies are currently underway. For the same reasons as for altimetry data, exploitable results (for the study of long term sea level variations) cannot be expected for several years.

Finally, measuring gravitational acceleration is another way to detect vertical movements of the Earth's crust. There are instruments capable of detecting a change in gravity, equivalent to a height variation of a few millimetres.

GLOSS

The Global Sea Level Observing System (GLOSS), an international program conducted under the auspices of the Intergovernmental Oceanographic Commission (IOC), aims at the establishment of high quality global and regional sea level networks for application to climate, oceanographic and coastal sea level research.

The main component of GLOSS is the 'Global Core Network' (GCN) of nearly 300 sea level stations around the world for long term climate change and oceanographic sea level monitoring. These stations provide sea level observations with centimetre accuracy, referenced in a global geodetic system.

The data are available via Internet from the University of Hawaii Sea Level Center, the Permanent Service for Mean Sea Level (PSMSL), and the World Ocean Circulation Experiment (WOCE).

SOERE SONEL

The SONEL project aims to produce mean sea level series and GPS solutions needed for studies on changing sea levels. This synergy should benefit tidal research on long-term variations in sea level. This project uses data on current and past water depths available on REFMAR.

SONEL was certified as a National Observation Service by INSU in January 2011. It was also recognized by the Environmental Alliance in July 2011 under the Long-Term Sea Level Variation System project. This SO INSU and SOERE project is supported by LEGOS with SHOM and the ULR. In November 2011, the GLOSS Group of Experts definitively agreed to make the GNSS component of SONEL the official "data assembly center" on this aspect of the program.

Thanks to the synergies between organizations running the SONEL system, the associated research activity will facilitate the dissemination of information on daily, monthly and annual mean sea levels and the associated co-located GPS data. Through the SONEL portal, mean sea level data can be disseminated for the purposes of the national strategy for sustainable management of the coastline, the strategic retreat and defence against the sea, ONERC indicators and the French Seas scoreboard proposed at the Grenelle de la Mer.

Different "mean levels"

The notion of mean sea level (MSL) of the observed height h (t) is ambiguous. It refers to a supposedly constant value. However, by definition MSL values are fundamentally​variable since they depend on both the central moment tn , the selected interval and the duration T of the interval: :

In addition, with respect to the tide, this ambiguity is compounded by the fact that the term "mean sea level" does not signify the mean of the observed heights h (t). The value is derived by filtering the data h (t) by the symmetric gate of width T, which is more complex than the arithmetic mean.

In tide gauging, mean sea level is determined by filtering the heights measured to eliminate the astronomical tide. This makes it possible to define different mean levels, including the daily mean sea level. In tidal period (TM1 ≈ 2TM2 ≈ 24.84 h), the simple mean of 24 hourly measurements leaves a residue containing tidal components that are problematic for statistical studies of mean daily sea level.

Calculating mean sea levels

The SONEL portal provides all the information needed to calculate daily, monthly and yearly mean sea levels according to international standards.